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IS straddles two countries - one which has invited intervention, the other which has not

As the US and allied Arab states launch air strikes on Islamic State (IS) positions in Syria and Iraq, questions arise as to the basis of such military action according to international law.

The legal evaluation differs where operations against IS in Iraq on the one hand and in Syria on the other are concerned.

The government in Baghdad has invited international forces to join in the fight against IS. Consequently, as long as this remained a campaign conducted within Iraqi territory, it may not have even been necessary to rely on an international right of collective self-defence.

In the exercise of its constitutional entitlements, the government of Iraq can use force internally to defeat an armed movement that has imposed itself forcibly upon a significant part of its territory.

It is true that a government is deprived of the entitlement to call in foreign military support where it has lost control over significant parts of territory and population due to a major public uprising against it.

A government so disenfranchised by its own population is not entitled to maintain itself in power through external armed intervention. For instance, the call of Ukraine's President Viktor Yanukovych for Russian armed intervention earlier this year was without legal effect. It occurred after he had lost the power to represent the state due to the public uprising against his rule.

In a similar vein, the West has criticised Moscow for continuing to direct military supplies to Syria's President Bashar al-Assad during the present conflict with the armed opposition.

Iran, too, has been condemned by the West for sending armed forces that had a key role in reversing the military balance on the ground in favour of the central government.

This is very different to the situation in Iraq.

IS is a sectarian movement imposing itself upon a part of a state through a military campaign.

It is alleged to maintain control over captured populations through terror, displacement, and murder of those deemed undesirable. It purports permanently to disenfranchise the local population in deviation of the international and constitutional human rights guarantees that apply to them.

The government of Iraq has recently been reconstituted, precisely to ensure that all segments of society in Iraq - Sunni, Shia, and Kurdish - can see themselves represented in it.

Its legitimacy has not been placed in doubt by the military successes of IS. To the contrary, the new government is positively obliged to do its utmost to deliver on the constitutional promise of human rights and defend its population from subjugation by IS.

It may receive any assistance in this campaign it requests. As long as the government in Baghdad assents, such assistance may also be directed at the Kurdish peshmerga forces in northern Iraq, who have had a key role in the campaign against IS thus far.

Self-defence?

The situation relating to operations against IS targets in Syria is more complex.

The Syrian government has not formally requested the assistance of the US.

Washington asserts that it informed Syria ahead of the launch of the strikes. Syria has responded in a muted way, indicating that it too is committed to the struggle against IS.

The failure to object more vigorously to the US action cannot in itself be taken to be equivalent to Syrian consent to the operation, although it is noteworthy when considered in the context of other developments. It is also interesting to note that Russia's objection to the operation has been unexpectedly moderate thus far.

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The Syrian government has lost all control over the parts of Syria held by IS

On the other hand, while many states, including the UK, regard the Syrian opposition National Coalition as the "legitimate representative of the Syrian people", these states have not recognised it as the Syrian government.

Hence, the coalition would not have the authority to consent to foreign military operations on Syrian soil.

A more successful argument would relate any action in Syria to the principal aim of assisting the government in Baghdad in the campaign to liberate its own territory.

It is not possible to defeat IS in Iraq unless the infrastructure supporting the veritable occupation of Iraqi territory by this post-modern, non-state actor that is also entrenched across the border, is addressed.

According to a ruling of the International Court of Justice in the 1986 Nicaragua case, where the US was found guilty of violating international law by supporting armed Contra rebels, self-defence could only be invoked by Iraq against Syria if IS acts as a direct agent of Damascus and under its operational control.

This is not the case. Instead, the Syrian government has lost all control over the parts of Syria held by IS.

Indeed, until very recently, it has made no attempt to dislodge it, leaving this task instead to the armed opposition groups. Damascus is manifestly unable or unwilling to discharge its obligation to prevent IS operations against Iraq from its own soil. Syria cannot impose the costs of its inaction or incapacity in relation to IS on neighbouring Iraq.

Hence, under the doctrine of self-defence, the zone of operations of the campaign to defeat IS in Iraq can be extended to cover portions of Syria beyond the control of the Syrian government.

In its letter to the UN Security Council of 20 September, Baghdad asserted that the IS infrastructure in Syria has made it impossible to defend Iraq's borders. It formally notified the Council that it had requested that "the United States of America to lead international efforts to strike Isil sites and military strongholds with our express consent".

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Thousands of foreigners have fought for Islamic State in Syria and Iraq

Three days later, when the strikes were launched, the US referred to this request, including its application to IS in Syria, to enable Iraqi forces to "regain control of Iraq's border".

That day, the UN secretary general appeared to offer support for this argument when he noted that the strikes "took place in areas no longer under the effective control" of the Iraqi government. At the high-level summit of the UN Security Council on terrorism a day later, the US action was not subjected to significant criticism, despite the fact that both Syria and Russia took the floor.

Overall, therefore, it is clear that forcible action in relation to IS in areas beyond the control of Syria can be lawfully undertaken in the exercise of the right of collective self-defence on behalf of Iraq to the extent necessary to secure its borders.

The argument of self-defence may be strengthened by the co-operation of key regional states in the operation against Syria. Jordan, another immediate neighbour, has pointed to incidents along its border involving IS, and is now pointing to the need to ensure its own security against further, and potentially worse, incidents.

However, in strict law, the right to self-defence of regional states would remain limited to what is necessary to terminate an on-going or imminent armed attack emanating from IS against its neighbours. This threshold does not appear to have been met as yet in relation to states other than Iraq.

A second strand of argument could relate to a claim of self-defence of the intervening states themselves.

The US and UK might argue that IS represents a manifest threat to their own security. However, according to Article 51 of the UN Charter, self-defence only applies to actual or imminent armed attacks, rather than potential or possible attacks.

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Military action is one part of a US-led strategy for a coalition to fight IS

In the wake of 9/11 and the loss of life and destruction caused, it has been accepted that even a non-state actor can launch operations sufficiently intensive to amount to an "armed attack"' in the sense of Article 51.

The US invoked self-defence in relation to its own armed forces based at Irbil in Iraq when threatened by an advance of IS. However, there are no such forces deployed in Syria.

It would be more difficult to point to any specific and imminent act of terrorism amounting to an armed attack against the US or UK emanating from IS in Syria.

The US did invoke its own right of self-defence, rather than a right derived from the threat posed to Iraq, in one instance. This concerned the Khorasan group affiliated with al-Qaeda and also based in Syria. Washington claims that this group represents an imminent threat of attack against the US and allies in accordance with Article 51. Hence it, too, was targeted.

Some lawyers have also pointed to the fact that IS has killed Western hostages and is threatening to kill more.

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France has joined the US in hitting IS in Iraq, but has ruled out air strikes on Syria

The recent murders of Western hostages are an outrage. Rescue operations specifically aimed at their release are clearly lawful. However, an assault on a small number of captured individuals, however gruesome, is not in law considered an armed attack against their home state capable of triggering a broader right to self-defence on the part of their home states, allowing them to launch the present campaign to degrade and destroy IS in Syria as such.

One might argue that IS represents a manifest and continuous threat of terrorism that is always imminent, given the virulence of the anti-Western ideology represented by it, and the capacity to plan and mount major terrorist operations at any time from the territory now controlled by IS.

This view would rather stretch the traditional understanding of the concept of self-defence.

The UN summit on terrorism has instead just unanimously imposed ever tighter obligations on states to prevent the formation and proliferation of armed terrorist groups.

Nevertheless, at its Wales summit of 5 September, Nato - a defensive alliance operating within the confines of Article 51 - declared with reference to IS that "if the security of any ally is threatened, we will not hesitate to take all necessary steps to ensure our collective self-defence".

Humanitarian intervention

Finally, it would be possible to base a claim for action on the activities of IS in Syria itself.

The Syrian government is under the obligation to secure its population from crimes against humanity committed on its territory. Clearly, it is unable to do so, having lost control over areas occupied by IS.

Hence, international action might be taken under the doctrine of humanitarian intervention to liberate the populations concerned from the grip of that movement.

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Kurdish Syrian refugees on the border with Turkey near Suruc

Forcible humanitarian action has been employed in more than 20 cases since the conclusion of the Cold War. However, in most of these instances, a UN mandate offered legal cover. A Russian veto would be likely in this instance.

Unilateral humanitarian action remains contested among states and legal scholars. However, the increasing emphasis on the rights of peoples and populations over the abstract doctrine of sovereignty makes it possible to justify forcible action in extreme circumstances of humanitarian need.

It is not yet clear, however, whether this threshold, which is higher than a string of severe human rights violations, has been met. Reports concerning an outpouring of refugees from IS areas into Turkey and other neighbouring states suggest that such a case may well be credibly made over the coming weeks.

However, in this instance the motivation of the intervening states may be in question. It may appear as if the actual interest of the intervening state lies in securing regional, and their own, security from a threat posed by IS, rather than in alleviating the plight of the local population.

It is also not clear who would take over if IS was forcibly displaced from parts of Syria - the government or the opposition, or even an international force for a short period? As was the case with the UN mandated operation to protect the Libyan population from the carnage unleashed by Col Muammar Gaddafi in 2011, the issue is likely to become intertwined with the broader campaign for "regime change" in Syria.

In sum, short of forming an unsavoury alliance with the Assad government, the strongest legal basis for action against IS in Syria is ancillary to the campaign now being conducted in Iraq. This may also extend to other affected states, such as Jordan.

'Danger of extermination'

The theatre of operations may extend to parts of Syria as may be strictly necessary to conclude that campaign successfully.

Self-defence on the part of the intervening states would require more specific indications of an imminent threat of terrorist operations emanating from IS in Syria and amounting to an armed attack against them.

In the alternative, action can be taken on behalf of the people of Syria that have fallen under the control of IS if there is an imminent danger of their extermination or forced mass-displacement, or of violations of their most fundamental human rights in a gross and systematic manner amounting to a massive crime against humanity.

However, forcible humanitarian action could then hardly be limited to those portions of territory under IS control. A broader solution to the conflict in Syria would need to ensue.

Marc Weller is professor of International Law in the University of Cambridge and director of the Lautperpacht Centre for International Law. He is the editor of the Oxford University Press Handbook on the Use of Force in International Law, which will appear this November.